The present invention relates to a method and arrangement for controlling a regulating variable which determines the rotational speed difference of a motor vehicle clutch. More particularly, the control method of the present invention is directed to controlling a regulating variable which determines a rotational speed difference of a clutch located in a force path between an internal combustion engine and a change-speed gearbox of a motor vehicle. The arrangement includes an electronic control unit configured and arranged to process input signals dependent on influencing variables specific to the motor vehicle and fixed required value characteristics and to provide output signals representative of required values for actuating an element adjusting the regulating variable. The arrangement further includes a device configured to record an influencing variable dependent on a speed of an accelerator pedal and operatively associated with the electronic unit, and a device configured to record an influencing variable dependent on a clutch temperature and operatively associated with the electronic unit. During a transient clutch condition, a phase with a variation, as determined by a specified time function, of the regulating variable is selectable, and a beginning and an end of both the transient clutch condition and the phase are respectively fixed as a function of the influencing variables specific to the vehicle.
In a known control method of the type described in EP 0 426 745 B1 for controlling a clutch which is arranged between a driving engine and a motor vehicle change-speed gearbox and which is configured as a lock-up clutch of a hydrodynamic unit or as a separating clutch, devices are provided for determining the engine rotational speed and the gearbox output rotational speed. A slip control switching device is also provided to determine the pressure for an actuating drive of the clutch or lock-up clutch. In this known arrangement, the control deviation is determined by comparing specified required slip values and the actual slip values determined from the rotational speeds, and this control deviation is converted into pressures in accordance with a specified control algorithm.
In gearbox controls, particularly on passenger car automatic gearboxes, the gearbox input rotational speed which coincides with the clutch output rotational speed is not usually known directly because no suitable sensors are arranged on the driven shaft of the clutch or, in the case of automatic gearboxes, on the turbine shaft of the hydrodynamic torque converter. In the known method, therefore, efforts are made to achieve good gear-changing quality without sacrifice of comfort and without the engine speeding up during gear changes of the change-speed gearbox even though the clutch output rotational speed is unknown. For this purpose, provision is made in the known method for the clutch output rotational speed, which is necessary for determining the slip, to be determined from a gear transmission ratio of the change-speed gearbox and the gearbox output rotational speed.
During a gear-changing procedure without a fixed gear transmission ratio in the known arrangement, activation of the clutch actuating drive then takes place with a dead time period during which the pressure is reduced until a predetermined slip is produced. During the adjoining second time interval which lasts until the instant when the new gear is selected, the pressure for the clutch actuating drive is kept constant or is varied while being controlled as a function of time. In this known method, therefore, the transient clutch condition is fixed by the beginning of the gear change and by the end of the gear change in the change-speed gearbox. The phase with the variation, determined by a time function, of the clutch actuating drive pressure is only optionally provided and, with respect to the beginning of the phase, is fixed by a predetermined slip value of the clutch and, with respect to the end of the phase, is fixed by the end of the gear change in the change-speed gearbox.
A different method for controlling a clutch in a hydraulic torque converter of a vehicle gearbox is described in DE 3640139 C2. This gearbox is connected to an engine with a throttle butterfly. The clutch is used under certain conditions for mechanically connecting the input and output ends of the hydraulic torque converter and is actuated, in specified vehicle speed range, by such a relatively small engagement force that clutch slip is possible. In this known method, a device for recording an influencing variable depending on the speed of the accelerator pedal is not used.
It is not possible with this second-mentioned known method to select a phase with a curve of the regulating variable determined by a specified time function in that known method is based on a different technical objectives. That is, due to clutch slip, known clutch systems do not achieve an adequate braking effect by the engine when the accelerator pedal is released for retardation purposes while the vehicle is being driven in a specified speed range in which the clutch is normally actuated in the slipping condition. Although effecting a control in the sense of a direct connection of the input and the output of the torque converter ensures the desired braking effect by the clutch during retardation, other problems can be caused under certain operating conditions.
In order to prevent such problems from occurring, the known method determines when the throttle butterfly has an amount of opening below a specified reference value and when the vehicle speed falls within the specified range. The engagement force of the clutch is then increased above the value which is normally present at the specified reference amount of opening of the throttle butterfly. Because in this known method the throttle butterfly opening falls below the specified value, the engagement force of the clutch can be abruptly increased when the accelerator pedal is released for retardation purposes in order to reduce or prevent slip so that a braking effect is caused by the engine in the desired manner.
An arrangement of a different type for controlling a clutch is described in PCT/WO 89/03318 A1, in which the clutch is located in the force path between a driving engine and a change-speed gearbox of a motor vehicle. The transmission capability of the clutch is automatically controlled as a function of the degree of opening or angle of the throttle butterfly, or as a function of the position of the accelerator pedal substantially such that the transmission capability is matched to the current torque requirement. A high transmission capability is therefore associated with a high torque requirement and a correspondingly lower transmission capability is associated with a lower torque requirement.
This known arrangement attempts to avoid torque shocks and oscillations in the drive train after the clutch in the case of a sudden increase in the torque requirement under tip-in conditions, i.e. during a change from instantaneous overrun operation in which the driving engine is driven by the vehicle (and of course the degree of opening of the throttle butterfly and therefore the transmission capability of the clutch are low) into driving operation in which the driving engine drives the vehicle. For this purpose, the transmission capability is increased in the known arrangement in this situation, i.e. in the case of a small throttle butterfly opening, only to the extent that the clutch operates with slip during a short interval of time. It is only when the throttle butterfly opening is very small and the speed of the throttle butterfly or of the accelerator pedal is very high that a phase with a falling curve, determined by a specified time function, of the transmission capability is initiated by the control. This phase, however, is located in time before the desired transient clutch condition characterized by slip. In this known arrangement, the curve of the transmission capability in the phase is always the same both with respect to the angle of inclination and with respect to the time duration even in the case of different throttle butterfly openings and even in the case of different speeds of the throttle butterfly or of the accelerator pedal.
A clutch control device for a motor vehicle shown in DE 3636952 C2 operates with an accelerator pedal pick-up and with a characteristic converter device activated by the accelerator pedal pick-up. This known clutch control device, in contrast to the other known arrangement for controlling a regulating variable determining the rotational speed difference of the clutch, does not employ a device for recording an influencing variable depending on the temperature of the clutch. Consequently, it is not possible to select a phase with a curve, determined by a specified time function, of the regulating variable. It is intended to offer the possibility of controlling the clutch engagement procedure as a function of the driving condition or non-driving condition of the vehicle and as a function of the accelerator pedal position. For this purpose, this type of known clutch control appliance is characterized by the accelerator pedal pick-up having a first pick-up for generating a first signal giving the accelerator pedal position and a second pick-up for generating a second signal giving the actuation speed of the accelerator pedal. The characteristic converter device has a first output device which responds to the first signal by generating a first actuation signal in order to carry out the complete engagement of the clutch device corresponding to the degree of actuation of the accelerator pedal, a second output device, which responds to the first and the second signal by generating a second actuation control signal in order to actuate the clutch device in a semi-engaged condition in relation to the degree of actuation of the accelerator pedal, and a third device which receives the first and the second actuation control signals in order optionally to supply either the first or the second actuation control signal, corresponding to the degree of actuation and the actuation speed of the accelerator pedal, to the clutch as a function of the presence of a plurality of specified conditions. On the basis of this configuration, the known clutch control device is intended to be capable of taking account of any given driving condition of the vehicle. That is, the clutch engagement procedure is intended to be carried out in an optimum manner depending on the current driving condition of the vehicle or on the vehicle being at rest and dependent on the accelerator pedal position of the vehicle.
Finally, DE 4100091 A1 shows an arrangement for monitoring a friction clutch, in particular an automated friction clutch of a motor vehicle, which includes a torque determination device for determining data which at least approximately represent the instantaneous torque occurring at the clutch, rotational speed sensors which supply data corresponding to the instantaneous rotational speed and the instantaneous output rotational speed, and a computing device which calculates data which depend on the data from the torque device and the rotational speed sensors, which follow one another at predetermined intervals and which represent the instantaneous friction power of the clutch at least approximately. The computing device adds the friction power data in order to determine a plurality of friction work averages over a plurality of time intervals of different durations compare the friction work averages determined in the individual time intervals with the specified limiting values allocated to the time intervals and, when the individual limiting values are exceeded, generates signals showing that the allocated limiting values have been exceeded. So that both an instantaneous peak load and the continuous load on the clutch can be monitored with a high level of accuracy, measures are taken in this arrangement such that a temperature sensor recording the temperatures of a component of the clutch is provided, and a plurality of limiting values, whose magnitude is a function of the clutch temperature, is respectively associated with the individual friction work averages. The computing device generates the signals showing that the limiting values have been exceeded as a function of the clutch temperature recorded and of the magnitudes of the limiting values specified for the clutch temperature recorded.
In order to reduce the energy consumption of motor vehicles with automatic gearboxes, clutches are used for locking up the hydrodynamic torque converter and, during certain gear changes in the automatic gearbox, these clutches must be disengaged. The lock-up clutches can be arranged in series with a mechanical torsional spring damper in the force path in order to keep torque oscillations caused by the internal combustion engine away from the downstream force transmission train. For this last mentioned purpose, it is also known to permit such clutches to operate with a certain slip, either as a supplement to or as a substitute for a torsional damper, in order to decouple the torque oscillations from the secondary side of the clutch.
An object of the present invention is to control the transient clutch conditions of such a clutch so that the transitions between steady-state and transient clutch condition satisfy high comfort demands (i.e. take place without being noticed by the driver) independent of whether the transient clutch conditions are initiated by gear changes, slip control processes or by other factors. These transient "opening" and "closing" clutch conditions occur in a clutch which only operates as a lock-up clutch for the torque converter during changes between the two steady-state clutch conditions of "clutch open" and "clutch closed". In the case of a clutch operating with a controlled slip, a third clutch condition, namely that with control to constant slip, is likewise considered as being "steady-state" so that the respective transient clutch conditions, appear during the change between, on the one hand, the third steady-state clutch condition and, on the other hand, one of the two other steady-state clutch conditions.
The present invention achieves the foregoing object by an arrangement and method in which a device configured to record an influencing variable dependent on rate of change of the clutch temperature, and at least one of the influencing variables, one of which depends on the rate of change of the clutch temperature and the other of which depends on the speed of the accelerator pedal, is utilized exclusively to fix the beginning and the end of the phase and to fix a curve, determined by a specified time function, of the regulating variable.
In the method and arrangement according to the present invention, various signals within an electronic gearbox control system influence the regulating variable variation, which is specified by a time function and specify this variation afresh in each computational cycle. The effects on the clutch of these changes in the regulating variable cannot then be noticed by the driver. That is, the transitions between the steady-state and transient clutch conditions take place at a high level of comfort.
In one embodiment of the method according to the present invention as applied to a friction clutch which can be engaged and disengaged by a clutch actuating element of the axial piston type, the working pressure, which here is to be regarded as the regulating variable, of the clutch actuating element is adjusted by a precontrolled pressure control valve whose pilot pressure is provided by a pilot valve electromagnetically controlled by an electronic control unit in the phase with a variation determined by a time function in accordance with the equation EQU P(t)=mp.times.t+po,
in which mp is the first time derivative of the pilot pressure p(t) (referred to below as the gradient), t is the time and po is the output pressure of the previous computational cycle. The change to the acceleration specification and the change to the temperature of the clutch are taken into account, as influencing variables specific to the vehicle, in the determination of the gradient mp.
If a vehicle acceleration is introduced by a change in the accelerator pedal adjustment, the condition of the clutch can be adapted previous to the change in engine torque. The gradient of the pressure variation is here calculated as a function of the rate of change of the acceleration requirement, i.e. throttle butterfly speed. Beyond a certain rate of change of accelerator pedal speed, the adaptation of the gradient additionally takes place beyond the actual actuation period. Corresponding to a recorded rate of change of the temperature of the friction surfaces of the clutch, the gradient can be influenced so that low rates of change reduce the gradient and higher rates of change increase the gradient. The temperature of the friction surfaces is determined, for example, empirically from a measured environment temperature, from a measured temperature of a working oil of a torque converter flowing around the clutch or from a different measured temperature which has a casual relationship with the temperature of the friction surfaces. In the case of a constant clutch temperature, however, no correction of the gradient takes place.
In the arrangement and method according to the present invention, therefore, the variation of the regulating variable specified by a time function (the characteristic phase of a transient clutch condition) can, for example, be specified as a function of the rates of change of different operating parameters and can be adapted afresh in each computational cycle. The algorithm thus established can be inserted as a sub-program within an overall clutch control algorithm which is employed for locking up a hydrodynamic torque converter and which operates by controlling a clutch slip to a predetermined required value.